This invention relates to compositions for co-dispensed gaskets and other objects and methods of manufacture thereof.
There is a need for gaskets, in the form of seals and other products, which can seal enclosure cabinets and electrical and electronic housings to provide protection against moisture and dust. Such gaskets are placed in covers and between frames, panels, and doors of electronic equipment, cabinets, and housings. Such gaskets may also need to provide an air and water-tight seal.
There are many applications in which outer layer(s) of a gasket and a core of the gasket require different and possibly mutually inconsistent properties, and thus proposals have been made for gaskets formed with an outer layer and a core of different materials.
For example, the prior art describes the production of prefabricated gaskets for electromagnetic shielding having an inner layer and an outer layer. The inner layer provides the gasket with physical properties such as compression deflection, tensile strength, and elongation. The outer layer provides the surface with properties such as electrical conductivity. Both the inner layer and the outer layer are elastomeric.
In these co-extruded compositions, the starting materials are single-component thermoplastic polymers. They are usually in the form of solid pellets, powder or granules. Heat is required and the solid is converted into a plastic molten state. The heat may be generated by both the screw and heating elements used in the co-extrusion process. Heating temperature varies depending on the melting point of the polymer. Generally the starting materials must be heated to above 100xc2x0 C. and usually much higher. The extruder screw delivers the molten plastic or rubber through a die. The plastic is heated sufficiently so that it will pass through the die and retain the shape imparted by the die. The die is the component that gives the shape to the thermoplastic mass. In most instances, this is entirely a thermoplastic process.
In the above, there are no chemical reactions taking place. The end product has the same chemical composition and cross-linking density as the starting material. Only the shape has been changed. After the part exits through the die, it usually undergoes a cooling stage so that it maintains its shape. Except for the final step, co-extrusion is very similar to injection molding. Instead of forcing the molten polymer through a forming die, injection molding forces the molten polymer into a closed mold. As soon as heat is removed, the polymer starts to solidify. In either case, the polymer must cool so that the extruded or molded part retains its shape.
In other instances, the one component solid polymer will contain reactive sites depending on the reactive component used. The reaction can occur in the presence of water, heat, UV or electron beam radiation as well as other methods. However, the retention of the shape imparted by the forming die is initially due to cooling of the molten polymer. The chemical reaction, if any, takes place afterwards.
It is common to co-extrude elastomers by this extrusion process. It is currently not possible to co-extrude a thermoplastic foam inside a thermoplastic elastomer. The thermoplastic foam must be extruded and cooled and the elastomer applied afterwards. Otherwise, the foam walls will melt and the foam will collapse when it contacts the hot, molten outer layer material. Heat activated and moisture-curing polymers can also be co-extruded by this process. However, here again it is not possible to co-extrude a foam inside one of these elastomers.
The above-described extruded products also cannot be formed-in-place. They can only be made in rolls or strips. After they are extruded, they must be cooled before they can be applied to a part. The degree of cooling is critical when application to thermoplastic materials is required. The extruded elastomer must be cooled below the melting point of the part it is being applied on or the part will melt or warp. Once cooled, they have a fixed shape and will not bond directly onto a surface without the use of adhesives. Furthermore, these materials cannot be formed-in-place because the ends cannot be attached onto each other without the use of external adhesives or by remelting the ends and fusing them together.
The two layers of an EMI gasket can be co-extruded such as described in U.S. Pat. No. 4,968,854, or the inner layer can be formed first with the outer layer applied afterwards as described in U.S. Pat. No. 5,141,770. The inner layer usually consists of a one-component thermoplastic resin or a one-component, heat-cured extruded rubber. The outer layer is also a one-component thermoplastic resin or a one-component, heat-cured extruded rubber. The outer component can also be made from a low viscosity coating dispersion containing an elastomeric binder, a metallic material, a curing agent, and a diluent, such as an organic solvent. The solvent is used to substantially reduce the viscosity of the coating, the inner layer being in this case extruded and solidified prior to the application of the coating.
Co-pending U.S. application Ser. No. 09/421,559, filed Oct. 20, 1999, which application is hereby incorporated by reference in its entirety, describes gaskets having a non-conductive substrate and a conductive outer layer. This gasket is prepared by applying the outer layer over the substrate (core) or by co-extruding the two components. However, these methods require the core to be at least partially if not fully cured prior to application of the outer layer. Or, if co-extruded, both the core and outer layer are at least partially cured and thus are not moldable or formable into a desired shape and thus are not formed directly on the part.
Thus, the above-described gaskets are prefabricated and cannot be extruded or dispensed directly onto the part to be gasketed.
The present invention is a further development of form-in-place and foam-in-place technology. It was discovered that at least two layer (core and outer layer) form-in-place and foam-in-place gaskets can be prepared by co-dispensing a core composition and an outer layer composition at ambient temperatures.
The present invention is directed to a gasket or other object comprising a core and a outer layer prepared by co-dispensing at a temperature of about 15xc2x0 C. to about 30xc2x0 C., a core composition and an outer layer composition, wherein the core composition is at least a two reactive-component foam or elastomer. The core composition may be a liquid or paste. The composition, when mixed together, has a viscosity at 25xc2x0 C. of less than 106 cps. When mixed together, the components react together to produce a solid thermosetting elastomer or foam.
The compositions to produce the gasket or other object of the invention are formulated to dispense through low pressure meter-mix dispensing equipment. By xe2x80x9clow pressurexe2x80x9d it is meant that the dispensing pressure is less than 1,000 psi. The nozzle of the meter-mix-dispenser may be attached to a robotic arm for form-in-place and foam-in-place applications.
Importantly, at least the core composition is a two reactive component polymer so that the viscosities of the starting materials will be low, the rate of crosslinking can be controlled by the formulator, and the curing rate of the core center and the surface of the core will be the same.
The core and outer layer compositions are applied in a non-solid state. By non-solid, it is meant that the viscosity of the composition at 25xc2x0 C. is less than 106 cps, preferably less than 105 cps. By solid, it is meant that the viscosity of the composition at 25xc2x0 C. is greater than 106 cps. The composition may also be applied in an uncured state.
In a preferred embodiment, the outer layer is at least a two reactive-component elastomer or foam. In a further embodiment, the core is a foam, and the co-dispensed core composition completes at least foaming prior to drying or curing of the outer layer composition.
The viscosities of the core and outer layer compositions vary from a thin flowable liquid to a highly thixotropic paste. If the core and outer layer compositions are compatible, the layers may flow into one another when they are co-dispensed. In this case, the core and outer layer compositions should be thixotropic with a thixotropic index (TI) of each greater than 3.0. If the two compositions are not compatible, lower viscosities and thixotropies may be used.
The present invention is further directed to a method for producing a gasket or other object comprising co-dispensing a core composition and an outer layer composition at a temperature of about 15xc2x0 C. to about 30xc2x0 C., wherein the core composition is at least one of an elastomer and a foam, comprises at least two reactive components, and has a viscosity at 25xc2x0 C. of less than 106 cps.
Preferably, the core composition and outer layer composition are dispensed through low pressure meter-mix dispensing equipment.
Further features of the invention will be apparent from the following description and examples.